The present invention relates to a harmonic drive apparatus and more particularly, to an improvement for a bearing member disposed between a flexspline and a wave generator disposed within the flexspline.
In a conventional harmonic drive apparatus such as disclosed in U.S. Pat. No. 3,285,099, a generally elliptical wave generator is rotated by its rotary shaft, and a flexible cylindrical flexspline is rotatably disposed outside the wave generator and has spline teeth around the outer circumference thereof. A circular spline secured to casings of the apparatus is disposed outside the flexspline and has spline teeth around the inner circumference thereof to locally engage with the outer teeth of the flexspline. The number of circular spline teeth is greater than the number of flexspline teeth. A bearing is disposed between the wave generator and the flexspline to rotate the flexspline. The bearing comprises inner and outer rings flexible in the radial direction thereof, and balls rotatably retained between the inner and outer rings. An output shaft is secured to the flexspline through a fastener such as a bolt to output the rotary force. The output shaft and the rotary shaft are rotatably connected through bearings to the respective casings secured to the circular spline.
In the above conventional harmonic drive apparatus, the wave generator is rotated by the rotation of the rotary shaft to successively move the engaging position in which the flexspline external teeth locally engage with the internal teeth of the circular spline. Since the number of circular spline teeth Nc is greater than the number of flexspline teeth Ns, the flexspline is slowly rotated in the direction opposite the rotational direction of the wave generator with a reduction gear ratio of (Nc-Ns)/Nc.
In this kind of the conventional apparatus, the bearing usually has a single circumferential race for the balls between the inner and outer rings. When the elliptical flexspline teeth locally engage with the circular spline teeth at the ends of the major axis of the flexspline, each of the two tooth engaging portions lies within some arc of .theta..degree. measured from the center of the flexspline. A stress is applied on the balls in the single circumferential line only when the balls are located in the vicinity of the tooth engaging positions. Namely, when the total number of balls is 23 and the arc angle .theta. of the tooth engaging portions measured from the center of the flexspline is 18.degree., the total number of balls stressed by tooth engagement is 2.3 from a calculation of 2.theta.=36 divided by the whole circumferential angle 360.degree. and multiplied by the total number of balls 23, and is thus only 1.15 balls for each tooth engaging portion. Accordingly, only one or two balls actually contribute to the engaging of the flexspline and the circular spline. Furthermore, the flexspline locally engages with the circular spline at the highest engaging pressure in the proximity of contact positions in which said one or two balls contact with the outer ring of the bearing. These contact positions are moved in the rotational direction of the rotary shaft as the balls are moved so that a slight torque fluctuation is generated in the flexspline. This torque fluctuation causes a vibration of the harmonic drive apparatus which is significant during low speed operation of the apparatus. Further, since the highest engaging pressure of the flexspline and the circular spline occurs in the vicinity of said contact positions as stated above, a greater stress is applied on the flexspline of a relatively thin wall at such places so that the flexspline is excessively worn away at such places and thereby damaged in a relatively short time span.